Many devices and most mobile computers today are equipped with wireless facilities for communication links. In order to guarantee for a reliable link, the quality of such a link needs to be observed. Several techniques for quality monitoring or estimating are known.
U.S. Pat. No. 5,151,902 describes a method for quality monitoring of at least two series connected transmission sections in a digital signal transmission link for digital equipment conforming to the synchronous digital hierarchy. Error message bytes in which parity errors are accumulated are transmitted in the section overhead of synchronous transport modules as special bytes for the accumulation of parity errors in successive transmission sections. A quality criterion for the monitored transmission link is acquired from a sequence of error message bytes.
The application EP 0 405 384 A2 discloses an apparatus for estimating communication link quality, wherein a communication link quality estimator is described and for a link a M-ary modulation scheme is employed. The link quality estimator is employed at the receiver, which includes a M-ary decoder responsive to received M-ary symbol channel bits for decoding the M-ary symbol into a set of M-ary decoded bits. The link quality estimator includes a time delay circuit for providing a version of the received channel bit M-ary symbol delayed in time by one symbol period. The link quality estimator converts the set of M-ary decoded bits into an auxiliary M-ary channel bit symbol representation in accordance with the M-ary modulation scheme, and compares on a bit-by-bit basis the delayed version of the received channel bit symbol and the auxiliary symbol representation. Detected errors are counted and averaged over a fixed block length to provide a bit error rate estimator.
In the application WO 98/38763 A1 is disclosed a system for an adaptive rate voice system. This adaptive rate system determines voice/channel coding rates, coding strategies and modulation/demodulation for voice quality and intelligibility. A system state estimator, channel status estimator, and channel status monitor provide feedback in the system. That means that a separate feedback channel exists between an adaptive transmitter and receiver. Only if the channel statistics have changed significantly, the channel status monitor computes a new set of critical operating parameters for the system. The system state estimator evaluates a system state indicator S(i) and if that indicator is valid, the critical operating parameters are compared to the previous one. Afterwards, a measure of speech quality is determined. The application describes a software implementation which is not suitable for a fast hardware implementation, since complex operations are necessary which lead to a considerable circuitry.
It is further known, that the transmission power in a wireless communication system can be controlled or adapted to improve the link quality, or that a link-quality depending modulation scheme can be utilized.
Although the present invention is applicable in a variety of communication links it will be described with the focus put on infrared link.
This patent application is related to the PCT Patent Application with International Publication Number: WO 97/25788, entitled “Robust method and apparatus enabling multi-mode wireless optical communication”, filed on Jan. 3, 1996, presently assigned to the assignee of the instant application and the disclosure of which is incorporated herein by reference. An optical communication system enabling communication between several co-existing transmitting and receiving stations is disclosed in this PCT Patent Application. In order to allow communication between the co-existing stations, a robust physical layer header is employed which can be understood by all participating stations. Information can be exchanged to allow negotiation and/or adaptation of the data rate used for transmission.
According to the Infrared Data Association (IrDA) recommendations for an Advanced Infrared standard, also abbreviated to AIr standard, a parameter is proposed that allows estimating the quality of a link. This parameter should give an indication of the quality of a link between a source and destination device, also referred to as transmitter and receiver. The physical layer design for an AIr system uses power-efficient L-slot Pulse Position Modulation (L-PPM) in conjunction with an adaptive variable data-rate transmission scheme. In a pulse position modulated signal, a single pulse of T seconds duration is always positioned in one of the L time slots of each L-PPM symbol. The variable data-rate concept has been introduced to improve the signal-to-noise ratio (SNR) under hostile channel conditions. Repetition coding is applied to reduce the rate and simultaneously provide a coding gain to maintain a sufficient bit-error rate in a noisy environment. Each L-PPM symbol is repeated RR times, where RR can be regarded as the rate-reduction factor. In the AIr system, typical values for RR are 1, 2, 4, 8, or 16, which correspond to the data rates 4, 2, 1, 0.5, or 0.25 Mb/s, respectively. Automatic adjustment of the data rate according to the prevailing channel conditions is highly desirable. This calls for an adaptive control of the transmission data rate by measuring a channel link-quality estimate at the receiver, determining from this estimate the required rate-reduction factor for future transmissions, and feeding a recommended parameter RR value back to the transmitter. All necessary operations such as measuring the link quality and deriving the recommended RR factor have to be performed fast to keep the loop delay low. A simple implementation is thus required, which eliminates the possibility to determine the link quality by measuring the signal-to-noise ratio (SNR) at the receiver's side.